A typical automotive engine-cooling fan assembly consists of one or more fans, each powered by an electric motor, and housed in a shroud which guides air through one or more heat exchangers. Each motor is typically supported by arms, or stators, which are supported by the shroud. Such a fan assembly can be placed upstream or downstream of the heat exchangers, which typically include both a radiator which cools the engine and an air conditioning condenser.
A fan assembly is required to efficiently provide the required amount of engine cooling while satisfying various noise criteria. These noise criteria usually concern both broadband noise and tones. At a given fan power, broadband noise is often found to be minimized by maximizing fan diameter, although fan noise, and particularly the tonal content of fan noise, increases if the fan overlaps an edge of the heat exchanger.
Fan efficiency is also often improved by maximizing fan diameter. One reason for this is that power expended in accelerating air through the fan is generally not recovered. This power is minimized by maximizing the fan area. Another reason is that a larger fan area provides better coverage of the heat exchangers. Due to the typically shallow depth of the fan assembly, the velocity of air through the heat exchanger cores outside the fan projected area is generally less than that inside the fan projected area. This flow non-uniformity increases the mean pressure drop, and decreases the effectiveness of the heat exchangers.
When the heat exchanger core area is approximately square, flow non-uniformity can be made reasonably small by using a single fan, the diameter of which is approximately equal to the length of a side of the square. When one dimension (typically the width) of the core is larger than the other dimension (typically the height), noise considerations generally limit the fan diameter to the smaller dimension. As a result, flow non-uniformity for a non-square core is generally larger than in the case of a square core, unless the aspect ratio of the core (the ratio of the longest side to the shortest side) is large enough to make practical the fitting of two fans side by side. Although the ideal core aspect ratio for a dual-fan assembly is two, such an assembly can offer a significant advantage in flow non-uniformity at smaller aspect ratios.
A measure of the extent to which a given fan arrangement provides good coverage of a heat exchanger core is the area ratio Af/Ac. This is the ratio of the total fan disk area Af to the area of the heat exchanger core Ac. For a single fan on a square core, the largest area ratio achievable without overlapping the edge of the core is xe2x96xa1/4, or 0.79. This is also the largest value achievable with a side-by-side dual-fan arrangement on a core of aspect ratio 2.
In practice, many automotive heat exchangers have an aspect ratio approximately midway between one, corresponding to a square and ideal for a single fan, and two, ideal for a dual fan. This presents a problem for the fan designer in that neither a single-fan configuration nor a conventional dual-fan arrangement has a favorable flow distribution through the cores. An aspect ratio of approximately 1.35 represents perhaps the worst case, where the ratio of fan area to core area is equally small for single-fan and dual-fan assemblies. Schematics of these two arrangements are shown in FIGS. 1a and 1b. For both single-fan and dual-fan assemblies, the area ratio is approximately 0.58. In fact, a 3-fan assembly, as shown in FIG. 1c, also has an area ratio of 0.58. Because the total fan area of the configurations shown in FIGS. 1a, 1b, and 1c are approximately equal, the efficiency and noise of these configurations will be approximately equal, as well.
In addition to the problem of maximizing fan area, another problem sometimes faced is maximizing fan power. This sometimes favors the use of multiple fans. In particular, in those cases where the largest motor available is too small to deliver the required cooling in a single-fan system, a multiple-fan assembly can be required, even at the expense of optimum system efficiency. This situation can be encountered when designing electric cooling systems to replace engine-driven fans for the cooling of light trucks. It also is likely to be encountered in the cooling of the new generation of fuel cell vehicles.
The present invention is an automotive engine-cooling fan assembly using two or more fans, where at least two fans overlap each other when viewed from the upstream or downstream direction. The set of arms which supports the motor driving one fan of an overlapping pair of fans is upstream of that fan, and the set of arms which supports the motor driving the other fan of the pair is downstream of that other fan. Overlap of a pair of fans can be demonstrated by projecting those fans onto a plane perpendicular to the axis of one or both of the fans. A first circular disk centered on the projection of the axis of one fan (the diameter of the first disk being equal to the diameter of that fan) overlaps a second circular disk centered on the projection of the axis of the other fan (the diameter of the second disk being equal to the diameter of that other fan).
In a preferred embodiment, the axial position of the set of arms which supports the motor driving each fan of an overlapping pair is substantially the same as the axial position of the other fan of that pair. The set of support arms for each fan of the pair excludes any members which would interfere with the placement of the other fan of the pair at the same axial location as that set of arms. This allows the module to be quite compact in the axial direction.
Preferably, a small clearance gap is maintained between said shroud and each of the fans along the portion of the fan""s circumference outside the overlap region. Preferably, a projection of the shroud opening, in a plane perpendicular to one or both of the fan axes is two generally circular elements that overlap in the region of fan overlap.
In a preferred embodiment, the two motors are approximately the same distance from the heat exchanger core. In this embodiment the distance between the core and the farthest point on one of the motors is between 0.8 and 1.25 times the distance between the core and the farthest point on the other motor. The length of the motors is often a limiting factor in making a fan assembly which is axially compact.
When a fan assembly according to a most preferred embodiment is viewed axially from upstream or downstream, the projected area of the set of arms supporting the motor driving one fan of the overlapping pair falls outside the projected disk area of the other fan of the pair.
In one embodiment the fan assembly is a dual-fan assembly. This arrangement provides good flow uniformity through a heat exchanger core in those cases where single-fan assemblies or conventional side-by-side dual-fan assemblies cannot, namely in those cases where the shroud covers a rectangular area of the heat exchanger core, and where the aspect ratio of that rectangular area is approximately midway between 1 and 2. In a preferred embodiment the assembly is a dual-fan system and is sized to move air through a core area with an aspect ratio of approximately 1.25 to 1.8. In a most preferred embodiment the assembly is a dual-fan system and the fan diameters are equal, or, if unequal, the fan diameter of the smaller fan is greater than 85 percent of the diameter of the larger fan, and the diameter of the larger fan is greater than 75 percent of the smaller dimension of the core area.
In other embodiments the fan assembly comprises more than two fans.
In preferred embodiments the extent of overlap, when measured in a plane which contains the rotation axis of at least one fan of an overlapping pair of fans and at least one point on the axis of the other fan, is greater than 10 percent of the diameter of the smaller of the two fans, and less than the blade span of the smaller fan. The diameter of the fan is taken to be the swept diameter of the fan blade tip, and the blade span is the radial distance from the hub to the fan blade tip. Overlap, diameter, and blade span are exclusive of any rotating tip band. Any greater overlap is likely to generate acoustic tones. The benefits of the invention will be relatively small if less overlap is used.
Although in some embodiments the fans are powered by hydraulic motors, in preferred embodiments they are powered by electric motors. One advantage of the invention is that it allows the use of two or more large motors in a relatively small package, where a side-by-side arrangement would limit the fan diameter to a size unable to absorb a large amount of motor power efficiently, and where an overlapping fan arrangement using two downstream or two upstream motor supports would be too deep to fit in the vehicle.
In preferred embodiments the shroud forms a plenum between the heat-exchanger core and the fans, and that plenum is deeper in those areas adjacent to fans with upstream support arms. This maximizes plenum depth for a given axial depth of shroud, and minimizes flow non-uniformity.
In preferred embodiments, recirculation is controlled by maintaining a small gap between each fan and the shroud in the non-overlapping portion of the fan circumferencexe2x80x94the portion which is not upstream or downstream of any other fan. This gap is preferably less than 2 percent of the fan diameter.
In preferred embodiments, banded fans are used. This type of fan, which has a rotating band attached to the blade tips, can maintain tip loading more effectively than a free-tipped fan in the overlap region.
The direction of rotation is specified relative to a viewer axially downstream of the assembly. In some embodiments one fan of an overlapping pair rotates in the clockwise direction and the other fan of the pair rotates in the counter-clockwise direction. This causes the fan blades to move in the same direction in the overlap region. This arrangement increases the total swirl velocity in the overlap region, reducing efficiency compared to the arrangement where the blades move in the opposite direction in the overlap region. However, the reduced relative velocity at the downstream fan can reduce fan noise relative to the alternative arrangement. Another advantage is that, due to the motor mounting arrangement, both fan motors have the same rotation direction relative to the motor. In some cases, identical motors can be used.
In other embodiments both fans of an overlapping pair rotate in the same direction (both clockwise or both counter-clockwise). This causes the fan blades to move in opposite directions in the overlap region. Due to swirl cancellation, this arrangement can be somewhat more efficient than the arrangement where the blades move in the same direction in this region. However, the increased relative velocity at the downstream fan can increase fan noise relative to the alternative arrangement.
In preferred embodiments the two fans of an overlapping pair have unequal numbers of blades. Also in preferred embodiments, the blade tips of at least one fan of an overlapping pair are variably spaced.
In preferred embodiments, both fans of an overlapping pair have blades which are forward-swept at their tips. This geometry has been found to have good efficiency as well as reduced fan noise relative to other geometries. This may be due to the fact that forward-swept blades have a relatively high tolerance of flow unsteadiness, such as that experienced when the blades move into, and out of, the overlap region.
In preferred embodiments, one fan of an overlapping pair rotates in the clockwise direction and the other fan rotates in the counter-clockwise direction, and the downstream fan has tip leading-edge sweep which is opposite the upstream fan tip trailing-edge sweep. In this configuration the downstream fan crosses the wake of the upstream fan in such a way that the unsteady forces on the different blade sections tend to cancel each other out, thereby reducing acoustic tones.
In other preferred embodiments, both fans of an overlapping pair rotate in the same direction, and the downstream fan has tip leading-edge sweep which is of the same sign as the upstream fan tip trailing-edge sweep. This is another configuration offering reduced tones.
In a most-preferred embodiment, the upstream fan of an overlapping pair has root trailing-edge sweep in the direction opposite the tip trailing-edge sweep.
The invention can be placed either upstream or downstream of a heat exchanger, or be placed between two heat exchangers.
The motors can be DC motors, and can be either mechanically or electronically commutated.
In preferred embodiments, the shroud comprises a barrel surrounding at least one of the pair of overlapping fans, and that barrel extends into the region upstream or downstream of the other fan of the pair, and contributes to the support of the mount of the motor of that other fan.